Science News

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Delta Science Fellow Examines How Mercury Enters the Food Chain

Allison Luengen in the field
(Photo courtesy of Allison Luengen)

In a winning Delta Science video, former Delta Science Fellow Allison Luengen, now an assistant professor at the University of San Francisco, introduces viewers to the two main sources of mercury in San Francisco Bay. Both hearken to the Gold Rush era, when about 10,000 tons of “quicksilver” were transported from the Coast Range to the Sierra to help miners capture the flecks of gold. Luengen’s research explores how this ongoing source of mercury enters the food chain.

“If you’ve ever been out on the beach on a windy day, you’ve seen dissolved organic matter in the foam of the waves,” Luengen said. “My research looked at how this dissolved matter affected the bioavailability of mercury.”

Bioavailability looks at how mercury goes from being dissolved in the water to moving into organisms. Mercury concentrations in fish are generally about a million times higher than concentrations in water so it’s important to understand how mercury moves from the water into an organism.

Luengen’s work is significant because it tries to understand the chemical and biological processes by which mercury enters the food chain. “In San Francisco Bay, we would like to ultimately have less mercury in the fish,” she said, adding that when she says mercury, she means methylmercury, which is the form that accumulates in fish.

The entire estuary and watershed are heavily polluted with inorganic mercury, much of which dates back to the time of the Gold Rush. Because there is so much mercury, cutting down on the amount that reaches the estuary (called the load) is difficult. Another way to approach the problem of mercury pollution is to understand what controls how mercury moves into organisms.

“The question of uptake is the focus of my research,” Luengen said. “Perhaps we can someday use what we know about uptake to limit the amount of mercury that gets into fish.”

If you don’t understand how mercury enters organisms, it is hard to think about how changing natural processes might affect mercury accumulation, she added. For example, a lot of wetlands are being restored in the estuary right now. Could that restoration affect the amount of dissolved organic carbon in the area?

“Quite possibly yes,” Luengen says. And then, based on her research, scientists would be able to predict how much mercury would be assimilated into the food chain as a result of a change in dissolved organic matter.

“The last thing we want to do in the estuary is to create an environment that would favor more mercury getting into fish,” she said. “We have to restore wetlands in such a way that we will not exacerbate the mercury problem.”

Methylmercury has no known biological purpose so it isn’t something algae would be expected to accumulate. But Luengen’s work and others have shown that algae do take up methylmercury and they want to know why since mercury getting into the algae is the first step in ultimately accumulating in fish.

Decreasing methylmercury exposure to people fishing in San Francisco Bay is a major goal, but there isn’t a lot of information on what mercury concentrations in marine organisms were like in the past. It has only been possible to measure methylmercury in the last couple of decades. And only in the last few years, have scientists had the technology to measure methylmercury in numerous samples. When Luengen was a graduate student, all of her measurements were done on samples that she processed by hand.

“We are moving away from that now,” she said, “so it’s a really exciting time for mercury research because we will be able to measure so many more things.”

Luengen’s research continues to focus on the cycling and bioavailability of metals, including mercury, in the San Francisco Bay-Delta. The daughter of a marina manager in Puget Sound, she grew up peering under docks to investigate “the many strange and wonderful creatures.” She earned a master’s degree in marine science and a doctorate in environmental toxicology from UC Santa Cruz.

In her spare time she enjoys sailing-“mostly small boats in more protected areas” but she has sailed offshore as well, from Santa Cruz to Santa Barbara.

“I have seen whales at very close range a couple of times,” she said. “Once, a grey whale surfaced within a stone’s throw of our 24-foot boat. I have never felt so small. You always hear that grey whales aren’t that big (at least for the whale family). Well, next to a small boat, they are enormous.”

Since 2003, the Delta Science Fellows Program, in conjunction with California Sea Grant, has paired graduate students and postdoctoral researchers with Bay-Delta agency scientists and senior research mentors in order to address research priorities and pressing science questions of the Bay-Delta system.